Integrated circuits are usually manufactured in large runs. However it is frequently desirable to make small runs of a specific integrated circuit, typically for prototyping. U.S. Pat. No. 4,924,287, the disclosure of which is incorporated herein by reference, describes a customizable integrated circuit. Methods for customizing of such integrated circuits are shown in U.S. Pat. No. 5,329,152, the disclosure of which is also incorporated herein by reference.
Customizable integrated circuits typically have predetermined portions which are adapted for modification before being supplied to the end user. Such modifications include, inter alia:
(a) electrically programing memory locations; PA1 (b) cutting conducting links; and PA1 (c) creating conducting links. PA1 (a) providing a plasma vacuum chamber; PA1 (b) placing the substrate (typically, a silicon wafer) on the cathode of the chamber; PA1 (c) introducing a plasma into the chamber, said plasma generated by applying Radio Frequency (RF) radiation to an active gas, the power density used is between 0.08 Watt*cm.sup.-2 and 1.59 Watt*cm.sup.-2 ; and PA1 (d) terminating the process when the PDP is deposited on the substrate to the desired thickness. PA1 (a) the material is stable enough to act as a passivation layer; PA1 (b) when ablated by laser, preferably a visible light laser, the material absorbs enough of the incident laser energy directed at it so that underlying layers are not damaged by laser energy; PA1 (c) only areas directly illuminated by laser energy are ablated; PA1 (d) the material is ablative, i.e., it vaporizes rather than explodes, so that very little debris is formed on the substrate when the material is ablated; PA1 (e) the material is ablated in response to relatively low levels of energy; PA1 (f) the material has good filling qualities, so that it can be deposited evenly over non leveled geometries; PA1 (g) the material is insulative; PA1 (h) the material adheres well to the substrate; PA1 (i) the material is resistant to at least some forms of metal etching; PA1 (j) the material is ablateable by e-beam; and PA1 (k) the material is etchable by some means, which preferably do not etch the structure of the underlying integrated circuit. PA1 providing an integrated circuit; PA1 depositing a PDP on the integrated circuit; and PA1 ablating the PDP at preselected locations using a laser beam, preferably a visible light laser beam.
Customization by cutting of conductor links is preferred since this method does not require extra circuitry on the integrated circuit as do electrically programmable logic devices. Furthermore, pre-produced links can carry a higher current density than created links.
Two methods are mainly used to selectively cut links. One method is to cut each link directly with a laser beam. However, direct cutting with a laser may require high laser energy densities. Application of large amounts of laser energy to integrated circuit surfaces may damage the integrated circuit.
A preferred method of customizing such circuits is to coat them with a layer of laser sensitive ablative material and to ablate the material at selected locations using a relatively small amount of laser radiation. After such ablation, the integrated circuit is etched using an etchant or other etching method that does not remove the ablative material, for example, by chlorine plasma etching. Thus, only areas previously ablated by the laser are etched. Customizable areas typically include metal links so that etching the links modifies the interconnections, and therefore the function, of the integrated circuit.
It is also known to use a photolithographic method wherein the integrated circuit is coated with a layer of radiation sensitive material and exposed to a pattern of ultra violet light, visible light, X-rays or to an electron beam. The coating material is developed and the areas exposed to radiation are removed. The integrated circuit is then etched as described above.
In practice, due to the characteristics required of them, very few materials are useful as laser ablative coatings. An efficient laser ablative material should be capable of absorbing a large portion of the laser energy and in response thereto be transformed directly and immediately to gas. Laser ablation sometimes causes the material to explode. Explosion transforms part of the material to gas, however, some of the material is also blown away as particles. Some of these particles may fall back on the chip and cover-up previously uncovered areas, counteracting the ablation/explosion at these areas. An effective laser ablative material should not form many particles. The term ablation means that the material is turned directly to gas, and very few particles are formed.
It is also desirable that the resultant ablation pattern be as close as possible to the irradiation pattern and that only small amounts of energy leak into the surrounding area and into the integrated circuit. Otherwise, the definition of the geometry will be poor and the integrated circuit may be damaged. Additionally, the material should adhere well to substrates and provide good coverage of step geometries used in microelectronic circuits. Since the purpose of the coating is to protect coated areas while etching the uncoated areas, it is important that the material be resistant to at least one method of etching, preferably a metal etching method.
An example of a material which has some but not all of the previous properties is Arsenic Sulfide. Arsenic Sulfide has most of the abovementioned properties, however, since it does not cover uneven surfaces very well it is not as useful as other materials.
Laser ablative materials which are ablated by ultra violet lasers are known in the art. For example, U.S. Pat. No. 5,302,547 shows covering an integrated circuit with a liquid polymer and ablating that polymer with ultra violet light. However, these polymers are transparent to visible light and are not known to be ablateable by visible light lasers.
Very few materials are known to be ablateable by visible light. Visible light is preferred to ultra violet light because laser technology supplies more efficient and less expensive lasers in visible light wavelengths.
U.S. Pat. No. 5,329,152 discloses the use of amorphous silicon as a visible light laser ablative coating material. Amorphous silicon is ablated by visible light lasers and is partially resistant to etching by chlorine plasma, which is used to etch metals. Thus, an integrated circuit with exposed metal links can be customized by using amorphous silicon as the ablative material.
One problem with amorphous silicon is its high vaporization temperature (2355.degree. C.)--1000.degree. C. over its melting point which increases the tendency to explosion and particle generation.
Plasma deposited polymers (PDP), which are described in "Plasma Polymerization", by H. Yasuda, Academic Press, Inc. 1985, have properties such as crack-filling, chemical inertness and selective permeability which make them useful for a variety of uses such as surgical prosthetics and semipermeable membranes. U.S. Pat. No. 5,320,875, 5,312,529, 5,283,119 and 5,308,649, the disclosures of which are incorporated herein by reference, disclose methods of manufacturing and uses of PDPs.
PDPs are typically manufactured as follows:
First, a substrate is placed in a plasma chamber. The chamber is then filled with a gas, such as methane, at a low pressure, typically on the order of 1 torr.
Plasma is then created in the chamber, typically using a radio frequency (RF) electric field which ionizes the gas. Consequently, a polymer layer is continually deposited on the substrate.
It should be understood that a PDP is not a direct polymer of the gas used in the process. It is believed that the gas breaks down in the plasma and gas precursors and their compounds form the PDP which is then deposited on the substrate (and on the walls of the chamber). The deposition process is a combination of two processes, one in which molecules hit the substrate and cling, and another in which they do not cling, and may even cause some material to be etched off the substrate. The temperature of the substrate dictates the types of molecules which are likely to cling to the substrate and the manner in which they will be attached to the PDP already deposited.
The gas usually flows through the chamber at a rate which determines the types of molecules that form in the plasma and, consequently, the type of PDP deposited. There are many other parameters which may affect the deposited PDP, such as the distance of the substrate from different portions of the plasma and the RF power used to create the plasma.
The gas used is typically an organic compound. However, some inert gases, such as argon, may be added in order to speed up the deposition process. It has also been observed that similar polymers can be created from different starting materials.